I was going through some old books to find things to take back to the Strand and I came across a book called Where Is Everybody?. It presents a number of different possible reasons why we haven’t found any evidence of alien life even though the universe is so big and so old that other advanced civilizations ought to exist. This is also known as Fermi’s Paradox.
Seeing this book again reminded me that in college I took an astronomy class and our professor asked us to try to come up with a reason to explain Fermi’s Paradox. Although I had entered school as an astronomy major, I was a Latin major by this point and was taking a lot of classes about Roman history. Because of what I was studying at the time I was coming at this from an unusual angle and I remember the professor mentioning to me that he hadn’t heard my explanation before (I can’t remember if the Where Is Everybody? book had this or not).
My basic point was that one of the big assumptions about this topic might be wrong. It is assumed that if alien civilizations have the ability to travel interstellar distances then they will be able to colonize all of the stars in the galaxy in a relatively short period of time. So, some people argue that there can’t be other advanced civilizations out there since we haven’t been colonized yet.
Using Rome as an example though, I think there is reason to think that there is a natural limit to how far any civilization can expand. If you look at a map of the empire at it’s height, it forms a rough circle centered on Rome. The Roman military could have gone into Scotland, Germany or other areas beyond their frontier, but for practical reasons the extent of their empire was fixed by the speed of travel and communication. If an emperor went to fight a campaign and got too far away from Rome, they became vulnerable to a foreign invasion or civil war since it would take so long to hear about it and then get back home to respond.
This same idea could apply to space faring culture as well. I think it is much more likely that advanced civilizations will stay centered in a small area where communications, travel and trade can happen relatively quickly (unless tesseracts or superluminal flight is possible.). It’s certainly appealing to think of a galaxy wide civilization, like Asimov’s Foundation series or Star Wars, but alien life is probably scattered in isolated pockets and separated by huge distances.